Disk drives are one type of magnetic storage device that employ MR heads. Digital information is stored within concentric tracks on a storage disk which is coated with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field.
During operation of a conventional disk drive, the disk is rotated about a central axis at a substantially constant rate. To read data from or write data onto the disk, a magnetic transducer is placed above a desired track of the disk while the disk is spinning. Writing is performed by delivering a write signal having a variable current to the transducer while the transducer is held close to the desired track. The write signal creates a variable magnetic field at a gap portion of the transducer that induces magnetic polarity transitions into the desired track which constitute the data being stored.
Reading is performed by sensing the magnetic polarity transitions on the rotating track with the transducer. As the disk spins below the transducer, the magnetic polarity transitions on the track present a varying magnetic field to the transducer. The transducer converts the varying magnetic field into an analog read signal that is then delivered to a read channel for appropriate processing. The read channel converts the analog read signal into a properly timed digital signal that can be recognized by a host computer system.
The transducer can include a single element, such as an inductive read/write element for use in both reading and writing, or it can include separate read and write elements. Transducers that include separate elements for reading and writing are known as "dual element heads" and usually include a magneto-resistive (MR) read element for performing the read function.
Dual element heads are advantageous because each element of the transducer can be optimized to perform its particular function. For example, MR read elements are more sensitive to small variable magnetic fields than are inductive heads and, thus, can read much fainter signals from the disk surface. Because MR elements are more sensitive, data can be more densely packed on the surface with no loss of read performance.
MR read elements generally include a strip of magneto-resistive material that is held between two magnetic shields. The resistance of the magneto-resistive material varies almost linearly with the applied magnetic field. During a read operation, the MR strip is held near a desired track, with the varying magnetic field caused by the magnetic transitions on the track. A constant DC current is passed through the strip resulting in a variable voltage across the strip. By Ohm's law (i.e., V=IR), the variable voltage is proportional to the varying resistance of the MR strip and, hence, is representative of the data stored within the desired track. The variable voltage signal (which is the analog read signal) is then processed and converted to digital form for use by the host.
There are many variables that can adversely affect the read performance of a magnetic disk drive. Of the variables, those which cause temperature variations in the MR element are particularly troublesome. More specifically, because MR elements are positive temperature coefficient devices, increases in the temperature of MR elements cause an increase in the resistance of the MR elements. Since the read signal (in volts) is proportional to the variations in resistance of the MR element multiplied by the bias current and since the bias current is a constant DC current, whenever the temperature of the MR element is increased, a thermal signal is generated which adds to the value of the read signal.
One of the variables which generates thermal signals results from the presence of foreign particles or other aberrations on the surface of the disk. These foreign particles and aberrations are known as asperities. Collisions between the asperities and the transducer cause the transducer to heat up. The increase in temperature resulting from the collisions between the asperities and the transducer causes an increase in the resistance of the MR element. Since the bias current is constant, the resulting voltage appears to be greater than the voltage that should be present based upon the data stored on the magnetic disk. The additive signal resulting from the increase in temperature of the M element is known as a thermal asperity.
Another variable which generates thermal signals results from the variations in the gap between the transducer and the disk due to the disk's surface variations. The head, because a constant current passes through it, is heated to a temperature above the ambient temperature (for example, 20.degree. above ambient temperature). The disk, because it has a temperature essentially equal to the ambient temperature, operates as a heat sink to take heat away from the head. When the gap varies (i.e., it is either greater than or less than a preset value) due to the surface variations of the disk, the head is either cooled or heated which causes variations in its resistance. Such variations are picked up by the read signal and are conventionally known as baseline modulation.
More specifically, for example, if a protrusion exists on the disk and causes the gap between the disk and the MR element to decrease, the disk operates as a better heat sink at that point on the disk which causes the temperature of the MR element to rapidly decrease. Because the MR element is a positive temperature coefficient device, as the temperature of the MR element decreases, the resistance of the MR element likewise decreases. Similarly, if a valley exists in the disk such that the gap between the disk and the MR element increases, the disk operates as a worse heat sink at that point on the disk which causes the temperature of the MR element to rapidly increase. Because the MR element is a positive temperature coefficient device, as the temperature of the MR element increases, the resistance of the MR element likewise increases. Accordingly, the MR element essentially becomes a microscope which maps the roughness of the disk.
The presence of thermal signals associated with thermal asperities and with baseline modulation can cause unwanted increases in bit error rates. In some instances, the increases in bit error rates are so great that they cause a severe data loss.
Because both magnetic data signals and thermal signals cause variations in the resistance of the MR element, in conventional systems, there is no clear-cut way to separate the desired magnetic data signals from the undesired thermal signals in the read signal. Accordingly, there is a need to develop a method and apparatus capable of distinguishing the desired magnetic signals from the undesired thermal signals so that the thermal signals (such as those associated with thermal asperities and/or baseline modulation) can be minimized or eliminated. The present invention is designed to overcome the aforementioned problems and meet the aforementioned, and other, needs.